Electrophoretic displays (EPIDS) are now well known. A variety of display types and features are taught in several patents issued in the names of the inventors herein, Frank J. DiSanto and Denis A. Krusos and assigned to the assignee herein, Copytele, Inc. of Huntington Station, N.Y. For example, U.S. Pat. Nos. 4,655,897 and 4,732,830, each entitled ELECTROPHORETIC DISPLAY PANELS AND ASSOCIATED METHODS describe the basic operation and construction of an electrophoretic display. U.S. Pat. No. 4,742,345, entitled ELECTROPHORETIC DISPLAY PANELS AND METHODS THEREFOR, describes a display having improved alignment and contrast. Many other patents regarding such displays are also assigned to Copytele, Inc. The following issued patents and pending applications by the inventors herein and assigned to CopyTele, Inc. and which may have some relevance to the present invention are: application Ser. No. 07/794,969, filed Nov. 20, 1991, entitled ELECTROPHORETIC DISPLAY PANEL WITH INTERNAL MESH BACKGROUND SCREEN; DUAL ANODE FLAT PANEL ELECTROPHORETIC DISPLAY APPARATUS, U.S. Pat. No. 5,053,763, issued Oct. 1, 1991; ELECTROPHORETIC DISPLAY WITH SELECTIVE LINE ERASURE, U.S. Pat. No. 5,066,946 issued Nov. 19, 1991; SEMITRANSPARENTELECTROPHORETIC INFORMATION DISPLAYS (EPID) EMPLOYING MESH LIKE ELECTRODES, U.S. Pat. No. 5,041,824 issued on Aug. 20, 1991; application Ser. No. 752,184 filed on Aug. 23, 1991 entitled ELECTROPHORETIC DISPLAY WITH SINGLE CHARACTER ERASURE, which is a continuation of Ser. No. 699,216 filed on May 13, 1991 having the same title; application Ser. No. 07/667,630, filed Mar. 11, 1991, entitled ELECTROPHORETIC DISPLAY PANEL WITH PLURAL ELECTRICALLY INDEPENDENT ANODE ELEMENTS, each of which are indicated below.
The display panels shown in the above-mentioned patents operate upon the same basic principle, viz., if a suspension of electrically charged pigment particles in a dielectric fluid is subjected to an applied electrostatic field, the pigment particles will migrate through the fluid in response to the electrostatic field. Given a substantially homogeneous suspension of particles having a pigment color different from that of the dielectric fluid, if the applied electrostatic field is localized it will cause a visually observable localized pigment particle migration. The localized pigment particle migration results either in a localized area of concentration or rarefaction of particles depending upon the sign and direction of the electrostatic field and the charge on the pigment particles. Certain of the electrophoretic display apparatus taught in such issued Patents are "triode-type" displays having a plurality of independent, parallel, cathode row conductor members deposited in the horizontal with transverse grid lines insulated from the anode lines. The grid and cathode lines form an X-Y matrix which can be accessed to address and control the pigment particles according to a selected X-Y intersection or pixel. U.S. Pat. No. 5,041,024 depicts a "tetrode" type display including a mesh electrode located between the anode and the X-Y grid cathode matrix. The operating voltages on both of the cathode and grid must be able to assume at least two states corresponding to a logical one and a logical zero. Logical one for the cathode may either correspond to attraction or repulsion of pigment. Typically, the cathode and grid voltages are selected such that only when both are a logical one at a particular intersection point, will a sufficient electrostatic field be present at the intersection relative to the anode to cause the writing of a visual bit of information on the display through migration of pigment particles. The bit may be erased, e.g., upon a reversal of polarity and a logical zero-zero state occurring at the intersection coordinated with an erase voltage gradient between anode and cathode. In this manner, digitized data can be displayed on the electrophoretic display.
An alternative EPID construction is described in U.S. Pat. No. 5,053,763, referred to above, which relates to an electrophoretic display in which the cathode/grid matrix as is found in triode-type displays is overlayed by a plurality of independent separately addressable "local" anode lines to constitute a "tetrode". The local anode lines are deposited upon and align with the grid lines and are insulated therefrom by interstitial lines of photoresist. The local anode lines may be in addition to a "remote" anode, which is a layer deposited upon the anode faceplate as in triode displays. The dual anode structure aforesaid provides enhanced operation by eliminating unwanted variations in display brightness between frames, increasing the speed of the display and decreasing the anode voltage required during Write and Hold cycles, all as explained in application Ser. No. 07/345,825, which is incorporated herein by reference.
In yet another form of EPID, as described in application Ser. No. 07/667,630, a triode is formed by a plurality of independent anode elements deposited upon one faceplate and a plurality of independent cathode elements deposited at right angles thereto on an opposing faceplate. An electrically continuous grid with a plurality of pores therein is deposited upon a layer of insulation overlying the cathode lines and is intermediate the cathode and anode lines.
To be useful as a display, an electrophoretic display must be able to assume a blank or erased state; must be able to display character data written during a write operation; and must be able to continually maintain or hold the written characters (and blank characters) in a hold mode until they are erased or overwritten. These three modes of operation, i.e., erase, write and hold are well documented in existing patents issued to the inventors herein and such description shall not be repeated at length herein. The above-described patents and pending patent applications provide apparatus and methods for hold, full write (all pixels written), full erase (all pixels erased), selective write (writing selective pixels in previously erased positions), selective character line erasure and selective character erasure. As one can ascertain, a character is generated by the activation or writing in of a plurality of pixels, each of which can form or provide part of the character. It is therefore an object of the present invention to provide a method for operating an electrophoretic display with an internal mesh element to allow single pixel erasure, thereby simultaneously realizing the advantages and benefits of a display of this type, as well as, the desired discrete erasure function.